A typical magnetic recording tape has a flexible plastic backing member carrying a layer of magnetizable particles in a polymeric binder. The tape may also have nonmagnetizable layers which likewise comprise inorganic particles dispersed in a polymeric binder. Other magnetic recording media are disks, drums, cards, etc., and may have rigid backing members. In order to apply either a magnetizable or a nonmagnetizable layer, the binder is usually dissolved in organic solvent which volatilizes. This is both wasteful and pollutes the atmosphere unless the solvent is recovered. The recovery of organic solvents is expensive and can involve danger of explosion and fire.
The aforementioned problems should be minimized by water-borne binders. However, other problems arise. For example, crosslinking agents, which usually are required to provide fully satisfactory physical properties such as good resistance to blocking and to wear, may react with water to form significant quantities of low-molecular-weight compounds that have deleterious effects. Some crosslinking agents have been suggested that are much less reactive with water, but these have required higher temperatures and/or longer times than are suitable for the manufacture of magnetic recording tape. It is not known whether any prior water-borne binder system avoids these problems and thus provides magnetic recording tape coatings having good physical properties such as resistance to blocking and to wear and acceptable magnetic recording properties.
A number of water-borne binder systems have been suggested for magnetic recording tape use, e.g., U.S. Pat. Nos. 3,023,123; 3,795,539; 3,901,816; Japanese Patent Publication No. J 51-103403 dated Sept. 13, 1976, of Hitachi Maxell Co.; and U.S. Defensive Publication No. T 946,007 (of Ser. No. 561,665 filed Mar. 24, 1975). Of these, only U.S. Pat. No. 3,901,816 and the defensive publication disclose water-borne binders which are crosslinkable. The defensive publication has only one working example of a magnetizable coating (Example 1), its binder comprising a copolymer of 85 parts of vinyl acetate and 15 parts of 2-ethylhexyl acrylate with melamine-formaldehyde condensate which is said to be an intra-molecular crosslinking component. However, the copolymer has no functional groups through which it could be crosslinked. If the copolymer contains functional groups such as hydroxyl, amine, carboxyl and epoxy groups, this is said to facilitate crosslinking. However, to so cross-link a melamine-formaldehyde condensate would require higher temperatures than can be tolerated by the most widely used backing members. The defensive publication gives no information concerning either the magnetic or physical performance of the resultant tapes.
We are not aware of any magnetic recording tapes on the market that were made using water-borne binders.